Adhesion and/or encapsulation of silicon carbide-based semiconductor devices on ceramic substrates

ABSTRACT

A SiC die with Os and/or W/WC/TiC contacts and metal conductors is encapsulated either alone or on a ceramic substrate using a borosilicate (BSG) glass that is formed at a temperature well below upper device operating temperature limits but serves as a stable protective layer above the operating temperature (over 1000° C., preferably &gt;1200° C.). The glass is preferably 30-50% B 2 O 3 /70-50% SiO 2 , formed by reacting a mixed powder, slurry or paste of the components at 460°-1000° C. preferably about 700° C. The die can be mounted on the ceramic substrate using the BSG as an adhesive. Metal conductors on the ceramic substrate are also protected by the BSG. The preferred ceramic substrate is AIN but SiC/AIN or Al 2  0 3  can be used.

RELATED APPLICATION DATA

This application is a continuation of copending U.S. patent applicationSer. No. 10/016,578, filed Nov. 1, 2001, now U.S. Pat. No. 6,911,714, tobe issued Jun. 28, 2005, which is a division of U.S. patent applicationSer. No. 09/351,106, filed Jul. 6, 1999, now U.S. Pat. No. 6,319,757,issued Nov. 20, 2001, which claims priority from U.S. ProvisionalApplication Ser. No. 60/092,039, filed Jul. 8, 1998.

BACKGROUND OF THE INVENTION

This invention relates generally to the adhesion and/or encapsulation ofsemiconductor devices or circuits on ceramic substrates, and moreparticularly to the adhesion and/or encapsulation of silicon carbidesemiconductors with metal or alloy contacts and conductors, and metalcircuits on AIN substrates.

SiC-based semiconductor devices possess tremendous advantages for hightemperature and high power solid state electronics. In addition, thesedevices offer potential advantages for high frequency and logic circuitapplications: e.g., power conversion (mixer diodes, MESFETs), and singlechip computers (n-MOS, CMOS, bipolar transistors). Non-volatile randomaccess memory SiC CCDs can hold charge for more than a thousand yearsthus, for example, making hard disks a thing of the past.

The potential maximum average power, maximum operating temperature,thermal stability, and the reliability of SiC electronics, for example,far exceeds Si or GaAs based electronics. The degree to which theseadvantages of SiC can be utilized, however, is presently constrained bythe thermal stability and electrical properties of the metal/SiCjunctions. The primary reasons for this are: (1) the power density ofSiC devices is limited by the thermal stability of the ohmic contactjunctions, and (2) substantial cooling is required to insure thestability of electrical contact junctions.

For a long time, researchers have been striving without success todevelop electrical contacts to silicon carbide to overcome theaforementioned constraints. Until these constraints are removed, SiCdevices/circuits offer only marginal, if any, advantages over Si andGaAs. Utilization of the full performance potential of SiC itself (forall devices), requires four types of performance-limiting electricalcontacts: (1) ohmic to p-type SiC, (2) ohmic to n-type SiC, (3)rectifying to p-type SiC, and (4) rectifying to n-type SiC.

The value of SiC electronics lies in its potential to extend thecapabilities of solid state electronics beyond what is possible with Sior GaAs. Thus, suitable electrical contact characteristics obtained inthe laboratory—under low stress conditions—must not drift or degrade,due to changes at the metal/SiC junctions, under actual device operatingconditions. This requires two additional attributes of metal/SiCelectrical contacts. First, the contact metal must form a junction withSiC that is chemically stable to ˜1000° C. (joule heating at highforward current densities in power SiC devices could easily causemetal/SiC junctions to reach such temperatures) or more. Second, thecontact metal (or metallization structure) must act as a diffusionbarrier to circuit and bonding metals (electrode metals) at the sametemperatures. Metal/SiC electrical contacts demonstrated previously donot come close to meeting all these stability requirements.

Commonly-assigned U.S. Ser. No. 08/612,216 Filed Mar. 7, 1996(counterpart PCT application PCT/US97/03497, filed Mar. 4, 1997), nowU.S. Pat. No. 5,929,523, which is incorporated herein by reference,however, discloses contacts of all four types which have the requisiteattributes. What is needed now is a suitable way to package SiC deviceswith Os and TiC/WC/W contacts and compatible conductive wires or lineson a suitable substrate, so as to be protected mechanically andchemically over a wide range of conditions. The current state of the artfails to meet this need.

Applicant has learned that a TiC contact on SiC does not, by itself,form a diffusion barrier to circuit or bonding metals. Appropriatecircuit/bonding metals such as W, Pt, Au and Pd, form intermetallicswith TiC. These solid state reactions change the composition of theelectrical contact junction, thus degrading it.

In a recent study, Tong et. al. (Ref. 1) examined the feasibility offabricating large area SiC substrates from CVD-grown SiC layers onSi(100) substrates via pressure bonding between two thermal oxidelayers. The process of Tong et al is as follows: (1) chemical vapordeposition of SiC layer on Si(100), (2) thermal oxidation to form a thinlayer of SiO2 on SiC layer, (3) fabrication of a separate Si wafer withthicker thermal oxide layer, (4) surface treatments to enhance thesubsequent pressure bonding, (5) pressure induced bonding between twothermal oxide layers, (6) high temperature annealing to improve bondingcharacteristics by densification, (7) lapping (polishing) to remove theoxide layer from the wafer initially with SiC layer, and finally, (8)chemical etching of exposed Si wafer down to SiC layer.

In addition to the complexity of the process of Tong, et al, the bondingbetween two thermal oxides is undoubtedly hampered by surface roughness,trapped bubbles, outgassing from the thermal oxides, and a thermalexpansion coefficient mismatch among Si, SiO2, and SiC (about 20%). Suchproblems become markedly more detrimental when the operating conditionsbecome harsher. Moreover, Si itself is not suitable for the hightemperature (over 250° C. and as high as 1150° C.). Harsh environmentapplications of SiC electronic devices, because of the thermal expansionmismatch, lower thermal conductivity, and lower melting temperature(1412° C.) of Si, will render the device structurally unstable.

In another bonding study (see Ref. 2), several systems-metals (Al, Ni,Ti, and W), polycrystalline silicon, phosphosilicate glass, and SiO2have been employed to bond polycrystalline SiC slabs together. However,high temperature annealing (1150° C.) under some controlled ambientconditions (e.g. steam with 2% HCl) was required to achieve effectivebonding. This anneal temperature is too high in that it exceeds thestability window of conventional Si devices. Problems associated withthe surface roughness of the slabs (as in Ref. 3) arise as well. Thebonding process using metallic layers at high temperatures can also leadto silicide formation. This is not suitable for SiC devices because, inthe anticipated operating ranges (temperature and environment), themetal layer could become unstable and/or the metal layer will beconsumed by the very bonding reaction with SiC which will change thecharacteristics of the SiC device.

In fact, this instability of a metal layer in contact with SiC at hightemperatures has been an obstacle to the development of SiC devices thatcan fully exploit the advantageous properties of SiC for hightemperature electronics. Similarly, the reaction bonding with thephosphosilicate glass poses the potential problem of phosphorousincorporation into SiC, which can change the electrical properties ofthe device. Thus, a need remains for a suitable SiC-to-substrateadhesion system.

The prior art also has not developed a material and/or process whichforms a stable, insulating, and adherent high-temperature (T>1000° C.)encapsulation layer on SiC devices and their related components(metallization on SiC die and AlN package substrate, connecting wires,and die insulating material).

For low-temperature electronic devices, the encapsulation layer isreadily formed using many different materials and processes. In the1970's a ceramic system, known as “CERATAB” packaging, was used. (Ref.4) In more recent years, the encapsulation materials have been siliconewith ceramic fillers (Ref. 5), epoxy resins (Ref. 6) and plastics (Ref.7). A more exotic approach to encapsulation consists of alternatinglayers of polyamide and ceramic. (Ref. 8) Organic encapsulationmaterials do not function at high temperatures. The high temperatureceramic materials suffer from high porosity and brittleness problemseven if the material itself satisfies the requirement for electricalinsulation and high temperature stability.

A recent patent discusses a fabrication process for a ceramic materialwhich consists of zirconia or hafnia powder suspended (encapsulated) incrystalline cordierite (mixture of MgO, CaO and Y2O3) glass-ceramicmaterials. (Ref. 9) Although this patent states that it is suitable inpackaging LSI circuits, the process as described, is not in fact,suitable for encapsulating integrated circuits because the fabricationtemperature is too high (840° C.<T<950° C.) for completed Si devices andcircuits to withstand. The use must be for fabricating ceramicsubstrates (on which encapsulated chips are mounted) because thezirconia and/or hafnia would strengthen the cordierite. In anotherapplication, phosphosilicate glass was used to encapsulate InP:Fe. (Ref.10) The phosphorous in this latter process was found to migrate from theencapsulation layer into InP:Fe.

For packaging SiC, the same problem would exist and would change theelectrical properties of the devices. A suitable encapsulation materialand/or process for the anticipated operating ranges of SiC devices doesnot exist. Such a need never existed before the advent of “stable” hightemperature SiC electronic devices. Now such a need exists, and no priorencapsulation material and method is known to be suitable.

REFERENCES

1. Q. Y. Tong, U. Gosele, C. Yuan, A. J. Steckl, and M Reiche, J.Electrochem. Soc. 142, 232 (1995).

2. P. K. Bhattacharya, Int. J. Electronics 73, 71 (1992).

3. T. Kamijoh, H. Takano and M. Sakuta, Jour. Appl. Phys., Vol. 55,(1984) 3756-3759.

4. J. Jablonski and W. Bielawski, Electronicka, No. 5, 221 (1972).

5. C. P. Wong and R. McBride, IEEE Transactions on Components, Hybridsand Manufacturing Technology 16 (8), 868 (1992).

6. R. Hunadi and N. Bilow, Proceedings of International SAMPE Symposiumand Exhibition: 1st International SAMPE Electronic Conference-ElectronicMaterials and Processes (Santa Clara, Calif., 1987) pg. 397.

7. C. P. Wong, Materials Chemistry and Physics 42, 25 (1995); J. A.Emerson, Proceedings of InterSociety Conference on Thermal Phenomena inthe Fabrication and Operation of Electronic Components (Los Angeles,Calif. 1988) pg. 190.

8. M. Tudanca, R. G. Luna, A. Fraile, J. Triana, J. M. Gonzalez, I.Vincueria, and C. Dominguez, Proceedings of 42nd electronic Componentsand Technology Conference, 120 (San Diego, Calif., 1992) pg. 120.

9. R. W. Adams Jr., D. R. Clark, L. L. Rapp, and B. Schwartz, Ausz. Eur.Patentanmeld. I 4(5) 426 (1988).

10. T. Kamijoh, H. Takano, and M. Sukuta, J. Appl. Phys. 55(10)3756(1984).

SUMMARY OF THE INVENTION

One object of the invention is to provide a suitable adhesion materialand method for adhering an SiC chip to a ceramic package substrate forhigh temperature operation.

Another object is to provide an encapsulation material and methodsuitable for packaging SiC chips and associated contacts and conductorseither alone or with circuitry on a ceramic substrate for stable hightemperature operation.

The process and system of the present invention, overcomes the problemsassociated with the prior art devices as previously described herein byproviding a semiconductor device having at least one siliconcarbide-containing layer which can be deployed in extreme thermalenvironments, while at the same time maintaining stable I/O electricalcontact with the device. Without this technology, lateral SiC devices(which require back-side electrical isolation) cannot be deployed inenvironments of high temperature, or rapid temperature changes.

This can be accomplished by providing a means of deploying SiC devicesand circuits and thin film thermocouples and transducers in harshthermo-chemical environments. More specifically, a stable, electricallyinsulating hermetic seal can be formed over the SiC-containing layerwith electronic devices and related components (metallizations, wires,etc. on the SiC-containing layer and on any underlying substrate).Therefore, this invention satisfies a long standing need for anon-conductive encapsulant that protects circuit components built on oraround SiC-based thin film sensors and circuits. It constitutes a hightemperature, non-corrosive environment encapsulant for circuitry basedon and around the SiC-containing layer. The thermal and mechanicalstability of the resulting hybrid structure does not limit thethermo-chemical or mechanical stresses to which devices including theSiC-containing layer can be subjected.

Furthermore, it is an object of this invention to provide a means ofdeploying SiC devices in harsh environments, by formation of a stable,insulating bond between SiC electronic devices and underlying ceramicsubstrates, such as AlN. The substrate is preferably polycrystalline AlNbut can be monocrystalline AlN, or a layer of AlN deposited on a SiCsubstrate.

This encapsulation method can also be used to package devices andcircuits on aluminum oxide substrates (Al₂O₃). For example, platinumresistive thin film devices on aluminum oxide can be encapsulated usingthe same composition and process. The need for such adhesives andprocesses arises from the packaging requirements for various SiC devicesunder development worldwide, as the performance potential of the SiCdevices is expected to extend the operating windows in temperature(caused by external temperature as well as by high-current-inducedheating) and chemical environments.

The subject invention includes a method for the protective encapsulationof a SiC-containing substrate and/or the adhesion of a SiC-containingsubstrate and an underlying ceramic package substrate. This methodcomprises encapsulating a SiC-containing semiconductor substrate havinga first and second major surface. It also can comprise encapsulating theSiC substrate on an underlying insulative package substrate (preferablypolycrystalline AlN but suitably monocrystalline AlN or a layer of AlNor SiC ceramic or other compatible bulk substrate materials), having afirst and second major surface. At least one of the first and secondmajor surface of the SiC-containing semiconductor substrate can becoated with a stable encapsulating layer which protects saidsemiconductor device from mechanical degradation and from thermaldegradation at temperatures above at least about 1000° C., preferably attemperatures above at least about 1200° C. Furthermore, at least one ofthe first and second major surface of said SiC-containing semiconductorsubstrate can be bonded with a bonding material to at least one of thefirst and second major surface of said underlying package substrate.This bonding material is preferably and advantageously the same materialas the encapsulating material. The bonded SiC-containingsubstrate-underlying ceramic substrate assembly or structure can beformed at temperatures of about 700° C., but, once formed, is stable tothe above described temperatures, i.e., at temperatures above at leastabout 1000° C., preferably up to temperatures above 1200° C.

The stable encapsulating layer is preferably formed by rapidly rampingthe temperature of the assembly up to about 700° C. Any metalliccircuitry located on the silicon carbide-containing layer or the ceramicsubstrate is protected against the effects of substantialoxidization/reduction thereof by formation of the stable encapsulatinglayer. Also, the SiC device bonded to the AlN substrate will beprotected against the effects of thermal shock temperature changes fromambient down to −200° C., and up to at least about 1100° C. by thestable encapsulating layer. Preferably, the coefficient of thermalexpansion of the stable encapsulating layer is near the coefficient ofthermal expansion of the carbide-containing semiconductor substrate. Thecarbide-containing semiconductor substrate preferably comprises contactsof WC and TiC, and/or Os, with conductors formed of any one of W, Au,Pd, Pt/Au.

The stable encapsulating layer is a form of glass which will protect thesurfaces and structures of the carbide-containing semiconductorsubstrate from mechanical degradation and from thermal degradation attemperatures to a range of at least about 1000° C., and preferably about1200 to 1250° C. Preferably, the glass employed in this invention is aborosilicate glass (“BSG”), and more preferably, the composition of theBSG comprises 30 to 50% by weight % of B₂O₃ and 70 to 50% by weight ofSiO₂. In any case, the stable encapsulating layer will in generalprotect metal contacts and circuits on any one of a silicon carbide, andoxide and nitride insulators, as well as metal circuitry on the ceramicsubstrate.

In the preferred form of the invention, the stable encapsulating layeris formed by applying an unreacted powdery material to one of the firstand second major surface of the silicon carbide-containing semiconductorsubstrate, and heating the unreacted powdery material to a temperatureat which it will form the stable encapsulating layer. Moreover, thestable encapsulating layer can be formed at a temperature of less thanabout 700° C., but once reacted is stable through the operating range ofthe SiC device, including temperatures substantially exceeding theformation temperature. The preferred powdery materials are a mixture ofprecursors for borosilicate glass, preferably B203 powder and SiO2powder, 325 mesh size. The powder mixture can be applied dry or in awater dispersion or paste (with a brief pre-heat to drive off theliquid).

It is also a desirable feature of the stable encapsulating layer that,when the bond is formed, the encapsulating material be non-invasive withrespective to the carbide-containing semiconductor substrate. Desirably,the stable encapsulating layer is preferably formed at a temperature ofless than 1000° C., and retains high viscosity to at least about 1200°C., and is immune to thermal shock to at least about 1100° C., and iselectrically insulating. The BSG encapsulant meets this constraint.

In the preferred form of the present invention, the SiC-containingsemiconductor substrate is an SiC die and the underlying substrate ispolycrystalline AlN. The expansion coefficients of AlN and SiCcomponents are nearly identical in this SiC/AlN structure, and that ofthe BSG encapsulant is close enough to both SiC and AlN so as to avoidseparation or cracking over a wide temperature range.

A method is also provided for the adhesion of a SiC-containing substrateto an underlying ceramic substrate, preferably AlN. The bonding oradhesive material generally comprises a similar composition to thestable encapsulating material discussed above, i.e., preferably aborosilicate glass, and more preferably a glass comprising 30 to 50weight % of B₂O₃ and 70 to 50 weight % of SiO₂. The bonding material ispreferably formed, as described above, by applying an unreacted powderymaterial, preferably in paste form, to one of the first and second majorsurface of said ceramic substrate, placing that surface against one ofthe surfaces of the SiC die, and heating the unreacted powdery materialto at least its bonding temperature, a preferred temperature of about700° C. within a range of 460° C. to 1000° C.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment of the invention which proceedswith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of a semiconductor device having atleast one silicon carbide-containing layer bonded to an underlyingceramic substrate made according to the present invention.

FIG. 2 is a cross-sectional diagram of a ceramic substrate having asilicon carbide-containing layer bonded to it and an encapsulatingcoating layer coated thereon according to the present invention.

FIG. 3 is a perspective view of a thermally sensitive SiC resistormounted in a flip chip manner on an AlN die and encapsulated accordingto the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The need for thermally stable ohmic and rectifying electrical contactsto n-type and p-type SiC and its solution described in commonly-assignedto pending U.S. patent application Ser. No. U.S. Ser. No. 08/612,216Filed Mar. 7, 1996 (counterpart PCT application PCT/US97/03497, filedMar. 4, 1997), now U.S. Pat. No. 5,929,523, incorporated by referenceherein. Osmium (Os) contacts exhibit the following properties:

-   -   1. Metal/SiC junction stable to over 1000° C.,    -   2. Protects its junction with SiC from circuit and bonding metal        diffusion (forms an electrically transparent diffusion barrier)        to >1000° C., and either    -   3. Forms an ohmic electrical contact to p-type SiC, or    -   4. Forms a rectifying (Schottky) contact to n-type SiC.

Osmium (Os) forms a rectifying (Schottky) metal junction on n-type SiCsemiconductor surfaces, which remains abrupt and firmly attached to atleast 1050° C. On P-type SiC, the Os layer forms an ohmic contact withthe lowest specific contact resistance of now possible with ohmiccontacts to p-type SiC. Stable, low resistance, p-type ohmic contactsare the key to development of all SiC bipolar high temperature powerelectronics.

Similarly tungsten/tungsten carbide (W/WC) forms an electricallytransparent diffusion barrier (ETDB) for TiC electrical contacts to SiC;(both p- type and n-type) where, the ETDB exhibits the followingproperties:

-   -   1. ETDB/TiC junction is stable to over 1000° C.,    -   2. Protects its junction with TiC from circuit and bonding metal        diffusion to over 1000° C.; thereby, also protecting the TiC/SiC        junction.

The electrically transparent diffusion barrier (ETDB) shields TiC/SiCjunctions from electrode metals to 1150° C., and forms a metallurgicaljunction with TiC that remains firmly attached and is stable to 1150° C.

These contacts enable fabrication of SiC devices and circuits that willnot degrade, or become unstable, under all strain conditions (thermal,electric field, mechanical) for which SiC itself can perform as asemiconductor device. They remove all restrictions imposed by metalcontacts on the thermal operating envelope of SiC solid state devicetechnology. In particular, they provide rectifying (Schottky) junctionsand ohmic contacts that will withstand sustained exposure to temperatureas high as 1150° C., and electron migration effects at high electricfields.

Based on the foregoing developments, a family of high temperature/powersemiconductor devices can be produced which satisfy a number of unmetcommercial needs. One immediate application is thermal sensors. The SiCdevices contacts can be used to make junction or resistive thermistors,that measure temperatures to 1922° F. (>1050° C.), and 2100° F. (1150°C.). This development permits SiC thermistors to compete, for the firsttime, with thermocouples, oxide resistors and pyrometers in thetemperature range of 300° F. to 2100° F. (150° C. to 1150° C.). It alsoenables the manufacture of hostile environment transducers, operable attemperatures above 460° F. (238° C.).

Other SiC devices that can now be made include Schottky diodes, PNdiodes and transistors as either discrete devices or as components ofintegrated circuits. Importantly, this technology opens up manypotential applications in the power conditioning and conversion field,such as power rectifiers for converting AC power to clean DC power.N—SiC Schottky rectifiers could handle powers nearly as high as the bestSi PN junction rectifiers, and PN junction SiC rectifiers could replacevacuum tubes. Other types of devices include bipolar transistors,thyristors, MOSFETs, MESFETs, IGBTs, and mixer diodes for use incommunications, as well as digital and analog to digital electronics.

All solid state semiconductor devices require one, or more, of the fourelectrical contacts discussed above. The invention of PCT/US97/03497(now U.S. Pat. No. 5,929,523) makes all four of these electricalcontacts possible, and facilitates connection to the devices to metalconductors including tungsten, platinum, palladium, gold, andplatinum/gold and palladium/gold, especially tungsten which readilyoxidizes at high temperature.

Protective encapsulation and adhesion of a SiC-containing substratecontaining such devices, contacts and conductors, and an underlyingsubstrate also containing metal conductors, can be provided employingthe method of the present invention. Typically, this method comprisesproviding a carbide-containing semiconductor substrate having a firstand second major surface and an underlying substrate having a first andsecond major surface. A bonding material is also preferably employed tobond at least one of the first and second major surface of theSiC-containing substrate to at least one of the first and second majorsurface of the underlying substrate. The bonded SiC-containingsubstrate-underlying substrate structure is formed without substantialmechanical or thermal degradation. Then, at least one of the first andsecond major surface of the SiC-containing semiconductor substrate iscoated with a stable encapsulating layer which protects thesemiconductor device from mechanical degradation and from thermaldegradation. Preferably, both the encapsulating and bonding functionsare performed using a single encapsulating-bonding material. Forexample, it has been discovered that a single glass compound,borosilitate glass (BSG), can be employed which provides the dualfunctionality by serving both as the encapsulating and bonding material.

The adhesive and encapsulation mixture is preferably initially composedof finely ground and mixed powders of the unreacted constituent oxidescapable of forming the glass material. It can be applied to the arearequiring adhesion of encapsulation as a dry powder, or as a slurry. Inslurry form, all vaporizable slurry constituents (e.g. water) should bebaked off before performing the forming process. The process isthermally activated by melting the B₂O₃ at T=460° C. The rate at whichthe glass forms is dependent on the ratio of B₂O₃ to SiO₂ in theunreacted mixture, how well the powders are mixed, and the formationtemperature employed. Rate is increased by more B₂O₃, finer and bettermixed powder, and higher temperatures.

The reaction from a powder or slurry to a solid glass is irreversible.Once formed, at temperatures of 460° C. up to 1000° C. (preferably 700°C.) the glass will not revert back to its original powdery or slurrystate, nor will it melt even at temperatures well over 1000° C., e.g.1200° C.

The reacting glass layer typically interacts adhesively with thesurfaces it contacts. For instance, the above-described glass materialcan bond with nitride surfaces in accordance with a parabolic ratelimiting reaction. The reaction region thickness is quite thin, and iscomposed of oxides, and in some instances nitrides. It remains stableand electrically insulating once formed. The encapsulant or adhesionlayer can also bond with SiC surfaces by forming an oxide (parabolicrate limited). Bonds can also be formed with SiC through an intermediateSi layer on the SiC surface. This intermediate layer is formed byoxidizing the Si layer and possibly some of the SiC surface, fusing withoxides with which it is in contact, and thereby forming a non-reactive,protective coating over metals and metal carbides.

EXAMPLE 1

As depicted in FIG. 1, a bonding material 10, comprising borosilicateglass (composition=30 to 50 weight % B₂O₃+70 to 50 weight % SiO₂), wasproduced and was employed as a high temperature, shock immune adhesiveto bond an SiC wafer 20 to AlN die 30. The resulting structure is a SiCsemiconductor on ceramic substrate 50.

The bonding material 10 was initially composed of finely ground andmixed powders of: (a) unreacted constituent oxides, or (b) reactedmixture. The adhesive was applied to one of the bonding surfaces(preferably the bottom side of the SiC wafer) as a dry powder, but canalso be applied in the form of a slurry. In slurry form, all vaporizableslurry constituents (e.g. water) should be baked off before performingthe bonding process.

The surfaces to be bonded (at least one of them coated with the driedadhesive) were placed together, and shift pressure was applied. Theprocess is thermally activated by transferring the assembly from ambientair to an oven preheated to a range of 700° C. to 800° C. This stepramped the temperature of the assembly up quickly (a few seconds atmost), from ambient to over 700° C. Reaction can be activated attemperatures as low as 600° C. for unreacted constituent powders, totemperatures above 1000° C. for already reacted powders. Using a fasttemperature ramp allows this reaction to be done in ambient air. The BSGreacts and protects metals before they can oxidize. Longer thermalreaction processes can be used if done in an oxygen free atmosphere.

Adhesive-AlN bonding occurs by chemical reaction. The reaction regionthickness is quite thin, and is composed of oxides and perhaps nitrides.It remains stable and electrically insulating once formed.

Adhesive-SiC bonding can be accomplished by one of three methods:

1. The adhesive is in contact with SiC surface so that the “bond” isformed by oxidizing the SiC surface.

2. The adhesive is in contact with a thin Si layer on the SiC surface sothat the bond is formed by oxidizing the Si layer and possibly some ofthe SiC surface.

3. The adhesive is in contact with a native grown oxide on the SiCsurface so that the bond is formed by fusion between the adhesive andthe native SiC oxide.

The completed preferred die bond structure 50 is shown in FIG. 1, and inencapsulated form in FIG. 2. Unique features of this structure 50include that: (a) it responds to mechanical and thermal stress as thoughit was a single piece, and (b) it is stable up to temperatures of atleast about 1100° C.

The structure 50 exhibited a high degree of thermal shock resistance.More specifically, the thermal shock resistance of structure 50 isthought to occur because: (a) the expansion coefficients of AlN die 30and SiC 20 wafer are nearly identical, (b) the expansion coefficient ofthe bonding material 10 is not much different from that of the AlN dieor SiC wafer, and (c) heat can be introduced into, or extracted from,the bonding material 10 at an extremely rapid rate because of the highthermal conductivities of SiC wafer 20 and AlN die 30.

EXAMPLE 2

The protective attributes of the stable encapsulating layer of thisinvention were experimentally determined, as follows:

Referring now to FIG. 2, a portion of a tungsten (W) wire 70, whichoxidizes easily, was connected to a metal bond pad 52 on an ohmiccontact on the SiC chip and to the metal bond pad 72 on the AlNsubstrate. The assembly 50 as well as the wire 70 and pads 52, 54, 72were then encapsulated within an encapsulating layer 80, on the AlN die30, following the temperature ramping procedure described in Example 1.Then the encapsulated assembly was heated to 1200° C. in air for 30minutes. As a control, a portion of the tungsten wire 70 was notencapsulated by encapsulating layer 80. In general, elemental tungstenforms two types of oxides WO₂ (purple) and WO₃ (yellow). The result ofthis experiment was as follows: (1) the exposed portion of the tungstenwire 20 was oxidized and formed a yellow, non-conductive powder (WO₃),(2) the encapsulated portion of the tungsten wire 20 remainedgrayish-silver (elemental, apparently unoxidized W) and its conductivityremained unchanged, no apparent reaction occurred between the tungstenwire 70 and either the encapsulating layer 80 on atmospheric oxygen.

Next, the AlN/encapsulant/SiC structure 60 after formation thereof wasinserted into liquid nitrogen, left in the liquid nitrogen for about 10minutes, and then removed therefrom. No changes were observed in themechanical properties of the structure 60. Finally, the aboveAlN/encapsulant/SiC structure 60 was placed in an 850° C. furnace for 30minutes, the temperature was increased to 1150° C. for 30 minutes, thetemperature was lowered to 850° C. for 30 minutes, and then structure 60was removed from the furnace. No change in the mechanical properties ofthe structure were observed.

FIG. 3 is a perspective view of an assembly similar to that of FIG. 2,showing the SiC die 20 mounted in a “flip chip” configuration, connecteddirectly to bonding pads 72, 74 on the AlN die 30 and covered by BSGencapsulant 80. Thus, the SiC wafer 20 can be adhered to a ceramicsubstrate such as AlN die 30, using BSG as an adhesive, or using BSG asan encapsulant, or both.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventioncan be modified in arrangement and detail without departing from suchprinciples. We claim all modifications and variation coming within thespirit and scope of the following claims.

1. A SiC semiconductor device comprising: a semiconductor substrate comprising silicon carbide having a first surface and a second surface, the substrate including a first doped region adjacent the first surface; a first conductive layer comprising metallic osmium (Os) contacting the first surface to form a first contact having an electronic interface to the first doped region; and an encapsulating insulative coating layer formed onto at least one of the first and second major surfaces of said silicon carbide-containing semiconductor substrate which protects said semiconductor device from mechanical degradation and from thermal degradation at temperatures above at least about 1000° C.
 2. A SiC semiconductor device according to claim 1 in which the first doped region is doped n-type and the first contact forms a Schottky junction.
 3. A SiC semiconductor device according to claim 1 in which the First doped region and a second doped region and intervening portions of the substrate are doped n-type, the first contact forms a Schottky junction and the second contact forms an ohmic junction, so that the device acts as a Schottky diode.
 4. A SiC semiconductor device according to claim 1 in which the first and second regions are doped to a first dopant type and an intervening portion of the substrate is doped to an opposite dopant type to form a first pn junction at an interface adjoining the first region and a second pn junction at an interface adjoining the second region.
 5. A SiC semiconductor device according to claim 1 in which the first region is doped to a first dopant type and the second region is doped to a second, opposite dopant type, and an intervening portion of the substrate is doped to form a first pn junction at an interface adjoining the first region, a second pn junction at an interface adjoining the second region and a third pn junction within the intervening portion.
 6. A SiC semiconductor device according to claim 1 including a layer of a protective metal covering the Os first conductive layer, selected from a group of conductive metals that is protective of the Os layer against oxidation and adapted to bond to an osmium surface.
 7. A SiC semiconductor device according to claim 1 including a layer of a protective metal selected from a group consisting of Pt, Pd, W, Au, PtAu, Ti, Zr, Hf, V, Cr. Fe, Ni, Cu, Nb, Mo, Tc, Ru, Rh, Ag, Ta, Re, Ir.
 8. A SiC semiconductor device according to claim 1 including a second doped region adjacent one of the first and second surfaces and a second conductive layer which comprises metallic osmium (Os) forming a second contact having an electronic interface to the second doped region.
 9. A SiC semiconductor device according to claim 8 in which the first and second regions and intervening portions of the substrate are doped p-type so that the first contact and the second contact each form an ohmic junction and the device acts as a resistive device.
 10. A SiC semiconductor device according to claim 8 in which the first and second regions and an intervening portion of the substrate are doped p-type so that the first contact and the second contact each form an ohmic junction and a gate structure overlies the intervening portion of the substrate so that the device acts as field effect transistor.
 11. A SiC semiconductor device according to claim 8 in which the first and second regions of the substrate are doped p-type so that the first contact and the second contact each form an ohmic junction, and an intervening portion of the substrate is doped n-type to form a pn junction with at least one of said first and second regions.
 12. A SiC semiconductor device according to claim 8 in which the first and second regions are doped p-type and the intervening portion is doped n-type and a contact coupled to the intervening portion comprises a material which forms an ohmic contact therewith so that the device is operable as a bipolar transistor.
 13. A device according to claim 1 including a ceramic substrate having first and second major surfaces, and an adhesive layer adhering one of the major surfaces of the silicon carbide substrate to one of the major surfaces of the ceramic substrate, the adhesive layer being stable to over 1000° C.
 14. A device according to claim 13 in which the coating layer is borosilicate glass.
 15. A device according to claims 14 in which the adhesive layer is borosilicate glass and the ceramic substrate includes a layer of AIN, SiC/AIN or Al₂O₃.
 16. A SiC semiconductor device comprising: a semiconductor substrate comprising silicon carbide having a first surface and a second surface, the substrate including a first doped region adjacent the first surface; a first conductive layer comprising a first contact metal layer on the first surface to form a first contact having an electronic interface to the first doped region, the first conductive layer including: a titanium carbide (TiC) layer contacting the second surface; a tungsten carbide (WC) layer covering the titanium carbide (TiC) layer; and a layer consisting essentially of elemental tungsten contacting the tungsten carbide (WC) layer; and an encapsulating insulative coating layer formed onto at least one of the first and second major surface of said silicon carbide-containing semiconductor substrate which protects said semiconductor device from mechanical degradation and from thermal degradation at temperatures above at least about 1000° C.
 17. A SiC semiconductor device according to claim 16 in which the first region is doped n-type, and the first contact forms an ohmic junction.
 18. SiC semiconductor device according to claim 16 including a layer of a bonding metal contacting the tungsten layer, selected from a group of conductive metals that is adapted to bond to a tungsten surface.
 19. A SiC semiconductor device according to claim 16 including a layer of a bonding metal contacting the tungsten layer, the bonding metal being selected from a group consisting of Pt, Pd, W, Au, PtAu, V, Ti, Zr, Hf, Cr, Fe, Ni, Cu, Nb, Mo, Te, Ru, Rh, Ag, Ta, Re, Jr.
 20. SiC semiconductor device according to claim 16 in which the first region is doped p-type, and the first contact forms a rectifying junction.
 21. A SiC semiconductor device according to claim 16 including a second doped region adjacent one of the first and second surfaces and a second conductive layer which comprises: a titanium carbide (TiC) layer on the one surface; a tungsten carbide (WC) layer on the titanium carbide (TiC) layer; and a metallic tungsten (W) layer on the tungsten carbide (WC) layer.
 22. A SiC semiconductor device according to claim 21 in which the first and second regions and intervening portions of the substrate are doped n-type so that the first contact and the second contact each form an ohmic junction and the device acts as a resistive device.
 23. A SiC semiconductor device according to claim 21 in which the first and second regions are doped n-type and a contact is coupled to an intervening portion of the substrate, the contact including a gate structure overlying the intervening portion so that the device is operable as a field effect transistor.
 24. A SiC semiconductor device according to claim 21 in which the first and second regions are doped n-type and a contact is coupled to an intervening portion of the substrate, the contact including a conductive layer contacting a surface of the intervening portion so that the device is operable as a transistor.
 25. A SiC semiconductor device according to claim 24 in which the intervening portion of the substrate is doped p-type so that the device is operable as a bipolar transistor.
 26. A SiC semiconductor device according to claim 24 in which the intervening portion of the substrate is doped n-type so that the device is operable as a MESFET.
 27. A SiC semiconductor device according to claim 24 including a gate contact structure comprising a third conductive layer overlying an insulative layer to form an insulated gate over the intervening portion so that the device acts as a field effect transistor.
 28. A device according to claim 16 including a ceramic substrate having first and second major surfaces, and an adhesive layer adhering one of the major surfaces of the silicon carbide substrate to one of the major surfaces of the ceramic substrate, the adhesive layer being stable to over 1000° C.
 29. A device according to claim 28 in which the coating layer is borosilicate glass.
 30. A device according to claims 29 in which the adhesive layer is borosilicate glass and the ceramic substrate includes a layer of AIN, SiC/AIN or Al₂O₃.
 31. A packaged SiC device which comprises: a SiC die having a silicon carbide-containing layer; a contact forming an electrical connection to an electronic device formed by at least one doped region in the SiC die, the contact comprising an electrically transparent diffusion barrier (ETDB) on the silicon carbide containing layer; a package substrate comprising a layer of SiC/AIN; a metal conductor on the package substrate connected to the contact; and a borosilicate glass layer adhering and interfacing the SiC die to the package substrate.
 32. A packaged device according to claim 31 which further comprises a borosilicate glass layer which encapsulates the SiC die and the metal conductor on the package substrate.
 33. A device according to claim 32 in which the SiC die is mounted on the package substrate in a flip chip configuration, in which the contact on the silicon carbide-containing layer is connected directly to a bonding pad on the package substrate.
 34. A device according to claim 33 in which the electronic device comprises one of a resistive device, a diode, a field effect transistor or a bipolar transistor.
 35. A device according to claim 31 in which the electronic device comprises one of a resistive device, a diode, a field effect transistor or a bipolar transistor. 